voip drawbacks.

Difficulty with sending faxes
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One drawback is the difficulty in sending faxes due to software and networking restraints in most home systems. An effort is underway to remedy this by defining an alternate IP-based solution for delivering Fax-over-IP, namely the T.38 protocol. Another possible solution to overcome the drawback is to treat the fax system as a message switching system which does not need real time data transmission - such as sending a fax as a email attachment (see iFax) or remote printout (see Internet Printing Protocol). The end system can completely buffer the incoming fax data before displaying or printing the fax image.

Internet connection requirement
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Another drawback of VoIP service is its reliance upon another separate service - an Internet connection. The quality and overall reliability of the phone connection is entirely reliant upon the quality, reliability, and speed of the Internet connection which it is using. Shortcomings with Internet connections and Internet Service Providers (ISPs) can affect VoIP calls. Higher overall network latencies can lead to significantly reduced call quality and cause certain problems such as echoing.
VoIP is not entirely reliant upon internet connections, however. VoIP systems can also utilize regular telephone lines and business-grade connections like T1 for voice service.[citation needed] A few business VoIP providers offer dedicated point-to-point T1 connections, not relying on an Internet connection for service. Although residential VoIP service typically uses only an Internet connection, business-grade VoIP service can use a variety of connection methods to provide ongoing phone service.
Many VoIP users still maintain a traditional analog voice line (business line) which allows them to dial emergency numbers and utilize a traditional fax machine.